This invention relates to an apparatus and a method for drilling a directional borehole in the ground.
Directional drilling is used for example for drilling oil and gas wells, boreholes for collection geothermal energy, or boreholes for installing a subterranean duct under a channel or other structure. With directional drilling a steerable drilling unit, or bottom hole assembly (BHA), is used for drilling the borehole. Typically, a BHA comprises a drive, a drilling bit and instruments for registration of the position of the drill bit. Several techniques can be used.
With the bent housing drilling technique, the BHA is provided with a bent housing such that the drill bit extends at an angle relative to the longitudinal axis of the BHA. This drilling position of the drill bit relative to the BHA is fixed. The BHA is connected to a drilling string for pushing the BHA forward and for rotating the BHA about its longitudinal axis. The drilling bit is typically driven by a mud drive. The drilling string is used for rotating the BHA such that the drill bit is positioned at the desired drilling direction. When the drilling string is stopped rotating, the BHA and its drill bit hold their position and the BHA drills a curved borehole. By rotating the BHA during the drilling process, the drilling bit is rotated about the longitudinal axis of the BHA and is thus not directed at any particular angle, therefore the BHA will drill a straight borehole.
An alternative is the rotary steerable technique. Also in this technique, the drill bit, mounted on a drill bit section, extends at an angled drilling position relative to a base section, also referred to as torque transfer section, of the BHA. However, the position of the drill bit relative to the base section can be adjusted, more in particular: the drill bit can be rotated about the longitudinal axis of the base section of the BHA while maintaining its angled drilling position relative to that longitudinal axis. The BHA is provided with a drive system for regulating the rotational speed of the drill bit section relative to the base section of the BHA. By continuously rotating the drilling position of the drill bit in a direction contrary to the rotation of the drill string and at the same rotational speed as the drill string, the drill bit is effectively kept in a constant drilling position relative to the borehole, and a curved borehole section will be drilled. When the drilling position of the drill bit is rotated at a rotational speed that differs from the rotational speed of the base section of the BHA, the drill bit is not kept in a constant drilling position relative to the borehole, thus, a linear borehole is drilled.
A major advantage of the rotary steerable technique over the bent housing technique is that the drilling string can be rotated while drilling a linear borehole. Rotating the drill string while drilling reduces the chances of the stick slip. Also, pushing the drill string forward takes less effort when the drill string is rotated at the same time. The rotary steerable therefore is a more efficient drilling technique.
With the rotary steerable technique, the drill string is rotated from the surface, and downhole devices cause the drill bit to drill the desired direction. Therefore, the BHA used in the rotary steerable technique is a more complicated than the BHA used with the bent housing drilling technique. With the rotary steerable technique, the BHA typically includes an internal orientating and control mechanism that counter-rotates relative to the drill string. This internal mechanism controls the speed at which the drilling position of the drill bit is rotated relative to the BHA, and thus controls the rotational position of the drilling position of the drill bit relative to the borehole. For example a controlled system is provided to control a mud drive, which mud drive rotates the drilling position of the drilling bit relative to the BHA and borehole. Typically, BHA comprises control valves that are used by a downhole control mechanism to regulate the flow of drilling mud passing through the mud drive, to thus regulate the rotational speed of the mud drive and control the position of the drilling bit, i.e. control the rotation of the drilling position of the drill bit.
The rotary steerable technique can be subdivided into two main groups regarding the way the drilling position of the drill bit is rotated relative to the BHA. The first group is the group of the point the bit systems, which use an internal eccentric part to position the drill bit section. The second group is the group of the push the bit systems, which systems use extendable pusher bodies, engaging the inside of the housing of the BHA or the wall of the borehole, to push the drilling bit in its desired position.
The rotary steerable technique can be used with a drilling string, composed out of drilling tubulars, and with a casing string, composed of casing tubulars. In the first case the BHA is connected to the surface with a drilling string, which drilling string is rotated to rotate the BHA. After the borehole has been drilled, the drill string and BHA are retracted form the borehole. The borehole is subsequently provided with casing.
With directional casing drilling, instead of a drilling string a casing string is used for pushing and rotating the BHA. The BHA is secured at, or partially in, the lower end of the casing string. After the drilling, the BHA is disconnected and retracted through the casing string. The casing string remains in the borehole to provide the borehole with a wall.
Even though the rotary steerable technique is more efficient, a drawback is that it requires the use of complicated control systems which allow for adjusting the speed and torque delivery of the drives in the BHA to control the rotational position of the drilling position of the drill bit section. This makes the BHA expensive and prone to failure due to the harsh conditions in the borehole.
The invention aims to provide a directional drilling system that is less complicated, and therefore preferably less prone to malfunction, than known directional drilling systems.